Process for making titanium dioxide

ABSTRACT

The disclosure relates to a process for making titanium dioxide, comprising: reacting titanium tetrachloride with oxygen by contacting the titanium tetrachloride with the oxygen in a vapor phase reactor under mixing conditions and at an elevated temperature to form a gaseous product stream containing titanium dioxide; separating the titanium dioxide from the gaseous product stream to form a process stream; analyzing the process stream to detect a concentration of titanium tetrachloride in the process stream; comparing the concentration of titanium tetrachloride detected in the process stream to an aim point concentration; and modifying the oxidation conditions to restore or maintain the concentration of titanium tetrachloride in the process stream at the aim point. In one embodiment, the process further comprises contacting the gaseous product stream with silicon tetrachloride under mixing conditions and at an elevated temperature to at least partially encapsulate the titanium dioxide with a silicon-containing compound and separating the at least partially encapsulated titanium dioxide from the gaseous product stream and analyzing the process stream to detect a concentration silicon tetrachloride for comparison to a silicon tetrachloride aim point concentration so that the conditions for silicon tetrachloride contacting can be modified to restore or maintain the concentration of silicon tetrachloride in the process stream.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosure relates to a process for making titanium dioxide. Thedisclosure additionally relates to contacting the titanium dioxide withsilicon tetrachloride to form surface-treated titanium dioxide. Yetadditionally, the disclosure relates to using an analyzer to control theconditions for making titanium dioxide or contacting the titaniumdioxide with silicon tetrachloride or both to improve the process byoptimizing process efficiency.

2. Description of the Related Art

The chloride process for making titanium dioxide pigment is well known.In the chloride process, TiCl₄ resulting from chlorination oftitanium-containing material such as rutile ore is oxidized to form TiO₂particles. The chloride process is described in greater detail in “ThePigment Handbook”, Vol. 1, 2nd Ed., John Wiley & Sons, NY (1988), theteachings of which are incorporated herein by reference.

A process for making a durable grade titanium dioxide pigment, withoutthe necessity of depositing surface treatments on the titanium dioxideby wet treatment has been described in U.S. Pat. No. 5,562,764 ofGonzalez and U.S. Pat. No. 7,029,648 of Subramanian et al. In thesedisclosures silicon tetrachloride is contacted with a titanium dioxidestream at one or more points downstream from the point where titaniumtetrachloride and oxygen are contacted to form the titanium dioxide. Theresulting titanium dioxide product has a surface treatment of asilicon-containing material which enhances durability.

In the oxidation of titanium tetrachloride to form titanium dioxide, itis useful to optimize the conditions to avoid having titaniumtetrachloride present in the product stream. The titanium tetrachloridein the product can lead to a variety of problems. In particular, ifwater is present in downstream operations it can convert titaniumtetrachloride to titanium oxychloride species which can cause severeprocessing problems including “bag filter blinding” in which titaniumoxychlorides clog filter pores in the pigment separation process, andblower fouling in which titanium oxychlorides accumulate on blowerinternals causing a lower compression ratio. In addition titaniumtetrachloride can cause jet pluggage in which titanium tetrachloride inthe feed jets cause poor gas distribution in the chlorinator.Additionally, the resulting titanium dioxide pigment product can havepoor acid solubility because soluble titania present in the product canimpair silica deposition during aqueous treatments to improve the acidsolubility properties of the pigment. To avoid these problems, operatorstend to utilize excessive heat during the oxidation step which iswasteful.

In the step of contacting the titanium dioxide product stream withsilicon tetrachloride it has been found that if the temperature is toolow silicon tetrachloride can remain unreacted. If the temperature istoo high the quality of the resulting at least partially encapsulatedproduct is poor either the encapsulation layer is fragile or manyparticles are not fully encapsulated leading to a product which lackssuitable durability.

The process of this disclosure overcomes these problems by integratingthe process with an analyzer capable of analyzing for titaniumtetrachloride or silicon tetrachloride or both.

SUMMARY OF THE INVENTION

The disclosure is directed to a process for making titanium dioxide,comprising:

(a) reacting titanium tetrachloride with oxygen by contacting thetitanium tetrachloride with the oxygen in an oxidation reactor underoxidation conditions to form a gaseous product stream containingtitanium dioxide;

(b) separating the titanium dioxide from the gaseous product stream toform a process stream;

(c) analyzing the process stream to detect a concentration of titaniumtetrachloride in the process stream;

(d) comparing the concentration of titanium tetrachloride detected inthe process stream to a titanium tetrachloride aim point concentration;and

(e) modifying the oxidation conditions to restore or maintain theconcentration of titanium tetrachloride in the process stream at the aimpoint.

In one embodiment, step (a) further comprises contacting the gaseousproduct stream with silicon tetrachloride under conditions effective forforming a treated product stream comprising titanium dioxide treatedwith a silicon-containing compound and wherein in step (b) the gaseousproduct stream is the treated product stream and the titanium dioxide isseparated from the treated product stream. In this embodiment,additionally, step (c) further comprises analyzing the process stream todetect a concentration of silicon tetrachloride in the process streamand step (d) further comprises comparing the concentration of silicontetrachloride detected in the process stream to a silicon tetrachlorideaim point concentration; and step (e) further comprises modifying theconditions for contacting the gaseous product stream with silicontetrachloride to restore or maintain the concentration of silicontetrachloride in the process stream at the silicon tetrachloride aimpoint. Yet additionally, in this embodiment step (e) further comprisesmodifying the mixing conditions or the temperature or both to reduce theconcentration of titanium tetrachloride in the process stream to reachthe titanium tetrachloride aim point.

In another embodiment, the disclosure relates to a process for makingtitanium dioxide, comprising:

(a) reacting titanium tetrachloride with oxygen by contacting thetitanium tetrachloride with the oxygen in an oxidation reactor underoxidation conditions to form a gaseous product stream containingtitanium dioxide;

(b) contacting the gaseous product stream with silicon tetrachlorideunder conditions effective for treating the titanium dioxide with asilicon-containing compound to form a treated product stream;

(c) separating the treated titanium dioxide from the treated productstream to form a process stream;

(d) analyzing the process stream to detect a concentration of silicontetrachloride;

(e) comparing the concentration of silicon tetrachloride detected in theprocess stream to a silicon tetrachloride aim point concentration; and

(g) modifying the conditions for contacting the gaseous product streamwith silicon tetrachloride to restore or maintain the concentration ofsilicon tetrachloride in the process stream at the silicon tetrachlorideaim point. In this embodiment, the process, additionally can compriseanalyzing the process stream to detect a concentration of titaniumtetrachloride; comparing the concentration of titanium tetrachloride inthe process stream to a titanium tetrachloride aim point concentration;and modifying the oxidation conditions to restore or maintain theconcentration of titanium tetrachloride in the gaseous product stream orthe process stream at the titanium tetrachloride aim point.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a simplified schematic flow diagram of the process of thisdisclosure.

DETAILED DESCRIPTION OF THE INVENTION

The titanium tetrachloride which is a starting material of the processcan be formed by chlorination of titanium-bearing material such asrutile ore. The starting material of the process can also include asource of one or more of aluminum, such as aluminum trichloride,silicon, such as silicon tetrachloride, boron, phosphorous, zirconiumand the like.

In the titanium tetrachloride oxidation, the titanium tetrachloride canbe preheated to a temperature of from about 300 to 650° C. and mixedwith aluminum trichloride forming a chloride mix which is fed into aheated stream of oxygen. This chloride mix may contain other metalcompounds used in the chloride pigment manufacture including compoundsof silicon, boron, phosphorous, zirconium, and the like. The titaniumtetrachloride and oxygen can react in a vapor phase reactor under mixingconditions and at an elevated temperature to form a gaseous productstream containing titanium dioxide. The vapor phase reactor can be atypical oxidation reactor as well-known in the art of titanium dioxidepigment production.

One or more phosphorous compounds can be introduced into the reactor.The introduction of one or more phosphorus compounds is generallypositioned to control corrosion and may be at some point downstream ofthe point where titanium tetrachloride and aluminum source areintroduced into the reactor.

For convenience the addition of the aluminum source can be in a mixturewith the titanium tetrachloride.

Oxygen can be present as an initial reactant. Although the process canbe run with the oxygen in excess of the amount required to oxidize thechloride mix, the process may be operated with the concentration equalto or less than the stoichiometric amount. A gaseous product streamcontaining titanium dioxide results from reacting titanium tetrachlorideand oxygen.

The titanium dioxide is separated from the gaseous product stream by anywell known titanium dioxide separation technique. The resulting processstream, from which substantially all if not all the titanium dioxide hasbeen removed, is analyzed as described herein to detect a concentrationof titanium tetrachloride in the process stream. The concentration oftitanium tetrachloride in the process stream is compared to a titaniumtetrachloride aim point concentration. Depending upon the results of thecomparison, to adjust the oxidation conditions to achieve the titaniumtetrachloride aim point, usually adjusting the mixing conditions or thetemperature or both in the step of reacting titanium tetrachloride withoxygen can restore or maintain the concentration of titaniumtetrachloride in the process stream at the titanium tetrachloride aimpoint. Techniques for adjusting the mixing conditions and thetemperature in order to restore or maintain the concentration oftitanium tetrachloride in the process stream would be apparent to thoseskilled in the art of making titanium dioxide. However, as an example,the temperature can be adjusted by heating or cooling one or more of thereactants and the mixing conditions can be adjusted by increasing ordecreasing the flow of one or more of the reactants.

In one embodiment, silicon tetrachloride-treated titanium dioxide can beformed in the process. In this embodiment, silicon tetrachloride can becontacted with the gaseous product stream typically under conditionseffective for treating the titanium dioxide with a silicon-containingcompound derived from the silicon tetrachloride, such as silica, andpreferably under conditions effective for at least partiallyencapsulating the titanium dioxide with the silicon-containing compound.Processes for treating titanium dioxide with silicon tetrachloride havebeen described in U.S. Pat. No. 5,562,764 of Gonzalez and U.S. Pat. No.7,029,648 of Subramanian et al. which are hereby incorporated byreference in their entireties. Treating titanium dioxide with silicontetrachloride in the gas phase can be useful to make durable titaniumdioxide.

For contacting the gaseous product stream with silicon tetrachloride, asuitable point of addition of silicon tetrachloride can be determined bydetermining when the conversion of titanium tetrachloride to titaniumdioxide is nearly complete or fully complete. For example, when at least97% of the titanium tetrachloride has been converted to titaniumdioxide. That is, the point where not more than 3% of the titaniumtetrachloride remains unreacted.

Additionally, the point of addition of silicon tetrachloride can bedetermined, when the present process is run with at least thestoichiometric amount of oxygen, using the following equations:

$K = \frac{\left\lbrack {{2\left( {{100\%} - u_{{TiCl}\; 4}} \right)} + {\varphi \times 100\%}} \right\rbrack^{2}}{u_{{TiCl}\; 4}\left( {\beta + u_{{TiCl}\; 4}} \right)}$and $T < {\frac{20733}{{\ln \; K} + 6.391} - 273.15}$

where

u_(TiCl4)=unreacted TiCl4(%)

β=excess O2(%)

φ=feed Cl2 mole ratio (mol/mol TiCl4), and

T=temperature (C)

K is the equilibrium constant for the reaction of the present process:

TiCl₄+O₂→Tio₂+2Cl₂;

Using this equation, one may calculate the point where the silicontetrachloride is first introduced from the feeds going into the reactor.Excess oxygen, β, is the oxygen in excess of that required to convertthe mixture of titanium tetrachloride and aluminum trichloride fed intothe reactor to their respective oxides (the stoichiometric amount). Thefeed chlorine mole ratio, φ, is the ratio of the moles of chlorine feddivided by the moles of titanium tetrachloride fed to the reactor over afixed period of time, for example, per hour. The percent unreactedtitanium tetrachloride, u_(TiCl4), is not more than 3% as is required bythe present invention. Using the calculated equilibrium constant, K, onecan then solve for the temperature at the point where silicontetrachloride is first introduced according to the present invention.The point in the reactor where this introduction is made according tothe present invention may be determined using the temperature profile ofthe particular reactor.

This calculation is independent of reactor size and pressure andrequires only knowledge of the feed composition (oxygen, chlorine andtitanium tetrachloride in moles per hour) and the temperature profilefor the reactor. Temperature profiles for a given reactor may bedetermined from well-known thermodynamic and heat transfer principles.

This method of calculating the addition points provide some flexibility,based on the feed mix that may be of importance in designing productfeatures to serve a particular pigment end use application.

The disclosure additionally relates to a process for making durabletitanium dioxide pigment by vapor phase deposition of surface treatmentson the titanium dioxide particle surface, the process comprising thesteps of:

(a) reacting titanium tetrachloride vapor and aluminum chloride and atleast a stoichiometric amount of oxygen in a reactor, typically in aplug flow reactor, to form a product stream containing titanium dioxideparticles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where at least 97% of the titaniumtetrachloride has been converted (3% unreacted titanium tetrachloride)to titanium dioxide.

The durable titanium dioxide pigment can be made by vapor phasedeposition of surface treatments on the titanium dioxide pigmentparticle surface and may also be operated with a mixture of titaniumtetrachloride and aluminum trichloride where the oxygen may be presentin an amount less than the stoichiometric amount. In this case theprocess comprises:

(a) reacting titanium tetrachloride vapor, an oxygen containing gas andaluminum chloride in a reactor, typically in a plug flow reactor, toform a product stream containing titanium dioxide particles; and

(b) introducing silicon tetrachloride into the reactor at one or morepoints downstream of the point where the titanium tetrachloride andoxygen were contacted and where the reaction temperature is no greaterthan about 1200° C.

In this case one would use the reactor temperature profile to locate apoint where the reaction temperature is no greater than about 1200° C.,and preferably no greater than 1100° C. The addition of silicontetrachloride would be made at this point or a point down stream of thiscalculated location. The use of the temperature profile and therequirement that the addition of silicon tetrachloride be made at alocation where the reaction temperature is less than about 1200° C. isuseful in cases where oxygen is present in excess, just equal to or lessthan the stoichiometric amount needed to oxidize the chloride mix.

The process of this disclosure can be used to make durable titaniumdioxide pigment. The term durable as used herein means a pigmentsuitable for exterior architectural coatings and automotive refinish orcolor coat or clear coat original equipment manufacturer finishes.Generally such pigments are characterized in that no more than about 25%of the pigment dissolves in sulfuric acid in the acid solubility test,and that silicon dioxide represents at least 1.4 to 2% of the totalweight of the pigment. The acid solubility test is described in Dieboldet al. “Rapid Assessment of TiO₂ Pigment Durability via the AcidSolubility Test”, JCT Research, July 2004 which is incorporated hereinby reference.

In one embodiment of the disclosure, the composition of the oxidetreatment deposited is a mixture of amorphous aluminum oxide andamorphous silicon dioxide. The thickness of the treatment layerdeposited in the present invention is not more than about 4 nm.Typically, the pigment is more than 99% rutile.

It is believed that the uniformity and the completeness of the surfacetreatment layer in the present pigments results in acid solubilities ofless that 25% even at silica concentrations of about 1% by weight of thetotal pigment.

Full, complete coverage of the particles means that the entire surfaceof the titanium dioxide particle is covered with the layer of surfacetreatment. The product of the present invention is characterized by thefact that at least 85% of the particles are fully and completely coveredby a layer of surface treatment. This layer is typically thin anduniform. The thickness of the layer can range from about 1 to 4 nm forparticles that are about 1% by weight aluminum oxide and 1.2% by weightsilicon dioxide. Higher concentrations of the surface treatment areexpected to produce thicker layers, but usually at equal uniformity.Microscopic analysis of the product has shown that about 80% or more ofthe pigment particles of the present invention have a treatment layerthickness of 1 to 2.5 nm, while in less than about 5% of the pigmentparticles, the treatment layer is about 4 nm thick.

Titanium dioxide can be produced by reacting a titanium dioxide ore withchlorine to produce titanium tetrachloride. Once the titaniumtetrachloride is purified, it is vaporized and reacted with oxygen at ahigh temperature to produce titanium dioxide. The titanium dioxide andchlorine are cooled and separated. Full conversion of titaniumtetrachloride to titanium dioxide is desirable. Incomplete conversion ofthe reaction may be caused by lack of oxygen, poor mixing between thetitanium tetrachloride and oxygen or low total heat available toinitiate the reaction.

An analyzer is used to detect titanium tetrachloride in the gas streamafter separating titanium dioxide and, optionally, after separating anytitanium oxychlorides or silicon oxychlorides. If titanium tetrachlorideis present the operator, which can be human or computer, may modify thereaction conditions to at least substantially eliminate the unreactedtitanium tetrachloride.

The operator may elect to modify the conditions by increasing ordecreasing the mixing of the oxygen and the titanium tetrachloride or byincreasing or decreasing the mixing of the gaseous first product streamand the silicon tetrachloride. This can be accomplished by increasing ordecreasing the flow of one or more of the reactants. It can also beaccomplished by adjusting process pressure or changing temperatures ofthe starting materials to the reactor. Additionally, it can beaccomplished by physically removing buildup in the reactor. Additionallyit can be accomplished by moving the point of addition of silicontetrachloride closer to or farther from the point where the titaniumtetrachloride and the oxygen are initially contacted.

The disclosure additionally relates to an in-process, real time controlloop capable of measuring titanium tetrachloride, silicon tetrachlorideor both remaining unreacted in the production of titanium dioxidefollowed by contacting the titanium dioxide optionally treated withsilicon tetrachloride to form surface treated titanium dioxide,comprising:

(a) providing starting materials selected from the group consisting oftitanium tetrachloride, silicon tetrachloride and mixtures thereof to avapor phase reactor;

(b) reacting the titanium tetrachloride with oxygen by contacting thetitanium tetrachloride with the oxygen under mixing conditions and at anelevated temperature to form a gaseous first product stream containingtitanium dioxide,

(c) optionally, contacting the gaseous first product stream with thesilicon tetrachloride, optionally at one or more points downstream ofthe point where the titanium tetrachloride and oxygen were contacted, bymixing the gaseous first product stream with the silicon tetrachlorideat an elevated temperature to form a gaseous second product streamcomprising silicon tetrachloride-treated titanium dioxide that istypically at least partially encapsulated by a silicon-containingcompound, the gaseous second product stream further comprising aquantity of gaseous titanium tetrachloride or gaseous silicontetrachloride or both;

(d) separating the titanium dioxide from the first gaseous productstream or the treated titanium dioxide from the second product stream orboth to produce a process stream that can contain a quantity of titaniumtetrachloride or silicon tetrachloride or both;

(e) measuring in-process the concentration of titanium tetrachloride orsilicon tetrachloride or both in the process stream;

(f) comparing the measured concentration of titanium tetrachloride orsilicon tetrachloride or both to that of the aim point concentrationsfor titanium tetrachloride and silicon tetrachloride; and

(g) adjusting the temperature for reacting titanium tetrachloride andoxygen or contacting the gaseous first product stream with silicontetrachloride or the mixing of the titanium tetrachloride with oxygen orthe mixing of the gaseous first product stream with silicontetrachloride to restore or maintain the concentration of titaniumtetrachloride and silicon tetrachloride at the aim points.

The aim point for titanium tetrachloride and silicon tetrachloride canbe the same or different and can depend upon the desired processoperation and suitable aim points would be apparent to a person skilledin the art of titanium dioxide manufacturing. However, the aim point fortitanium tetrachloride, silicon tetrachloride or both can be zero.

The process of this disclosure can use an analyzer for determining thepresence of titanium tetrachloride, silicon tetrachloride or both in theprocess stream which results from separating the at least partiallyencapsulated titanium dioxide from the treated product stream. Even with3% unreacted titanium tetrachloride in the process stream, theencapsulation of the titanium dioxide with silicon dioxide can befurther enhanced by operating at the point where a minor amount ofunreacted silicon tetrachloride is present, typically where an amount ofless than about 10% of silicon tetrachloride is present based on theentire amount of silicon tetrachloride.

In the present process the concentration of titanium tetrachloride orsilicon tetrachloride or both can be measured by an optical methodselected from the group consisting of transmission filter Infraredspectroscopy, transmission Fourier Transform Infrared spectroscopy,Raman spectroscopy, Near Infrared Spectroscopy and Ultravioletspectroscopy. The measurement of the concentration can be made in afrequency range selected from 200 nm to 400 nm, 12,500 cm⁻¹ to 4000cm⁻¹, and 4000 cm⁻¹ to 400 cm⁻¹.

For accuracy and precision, the presence and the concentration of atleast one of titanium tetrachloride or silicon tetrachloride can bemeasured by Transmission Gas phase FTIR 619 cm⁻¹ (SiCl₄) and 499 cm⁻¹(TiCl₄). For diagnostic information about the process, the quantity ofcommon combustion products such as HCl, CO₂, and NOCl can be measuredusing FTIR. Suitable Process FTIR equipment are commercially availablefrom any number of analyzer vendors such as Hamilton-Sunstrand.

The term “real-time” means actual time when at least one of the steps ofreacting titanium tetrachloride and oxygen or contacting the titaniumdioxide with silicon tetrachloride are taking place in the process.Thus, the in-process, real-time control loop of this disclosure providesa way to optimize oxidation reaction conditions without excess heat orraw materials such as, without being limited thereto, oxygen. Moreover,the disclosure provides a way to optimize the contacting conditions toavoid unreacted titanium tetrachloride or silicon tetrachloride or bothin the product. The process can improve the titanium dioxideencapsulation with a given quantity of silicon tetrachloride.

The analysis is usually conducted on the cooled gaseous product streambut it could be conducted on the gaseous product stream after thebaghouse but before the stream has cooled.

The entire content of the gaseous product stream can be analyzed, forexample using a Raman spectroscopy in-line analyzer. Alternatively, anextractive sampling technique can be used in which a sample of thegaseous product stream is removed from the process, filtered, andtransferred to the analyzer, typically, by means of a heated sample lineusing, for example, FTIR spectroscopy. In the extractive samplingtechnique, the sample is usually temperature and pressure adjusted forthe analyzer. Once the analysis is conducted on the sample the samplecan be sent back to the process or sent to a scrubber. Alternatively, asingle detector may be located downstream from both the first zone andthe second zone preferably downstream of the gas-solid separationequipment. Its actual location is not critical as long as it is locatedin an area where the temperature ensures that titanium tetrachloride andsilicon tetrachloride will be in the vapor phase. Measurement of theconcentration of titanium tetrachloride and silicon tetrachloride can bemade following in-line filtering or screening out of interferingparticles or other materials.

In general the operation of the present process may be described by atypical process control loop, somewhat similar to the kind disclosed inU.S. Pat. No. 6,562,312, which is shown the FIGURE. The control loopcomprises a control devise, a feedback controller, and an analyzer. Theanalyzer measures the concentration of at least one of titaniumtetrachloride and silicon tetrachloride downstream. Typically anyoxychloride species resulting from partial or incomplete oxidation ofthe starting materials such as titanium tetrachloride or silicontetrachloride are removed before the analyzing step. The analyzerproduces an output signal representing the measured concentration of atleast one of titanium tetrachloride and silicon tetrachloride. Thisoutput signal is sent to the feedback controller. There theconcentration of the titanium tetrachloride and silicon tetrachloridemeasured by the analyzer is compared to the aim point which is usuallypredetermined. The feedback controller, based on this comparison,provides input to the control device, shown in the FIGURE as a flow/heatcontrol device for adjusting the flow or the heat of the reactants orboth. The flow/heat control device can adjust one or more of thetemperatures and the mixing conditions of the oxidation reactor ordownstream of the oxidation reactor, shown in the FIGURE as theoxidation reaction zone, to restore or maintain the concentration of thetitanium tetrachloride and silicon tetrachloride at the aim point.

Algorithms used in the analyzer and the feed back controller to convertthe data collected to a signal output are not critical. One skilled inthis art can select or design an algorithm suitable to the specific typeof analyzer or feedback controller. The control device may be anyregulated flow device equipped with an automatic actuator. Typically thecontrol device is a valve.

The present control loop responds rapidly to variations in temperaturesand mixing indirectly by comparing the concentration of titaniumtetrachloride and silicon tetrachloride present at a certain time withthe concentration selected as the aim point. It is desirable to set theaim point at the lowest reliable concentration of titanium tetrachlorideand silicon tetrachloride that is detected by the analytical detectiondevise used in the control loop.

Any suitable analytical detection method may be used in the process asdiscussed herein above. Using the techniques disclosed herein, titaniumtetrachloride in concentrations as low as (10 ppm) can be measured.Using the techniques disclosed herein silicon tetrachloride inconcentrations as low as about 10 ppm can be measured.

The process of this disclosure provides a real-time, in-process controlat an aim point. The present control loop that is fast and is responsiveto the demands of continuous in-process operation.

The disclosure can provide a process with lower Cl₂ yield loss due tolower fuel consumption in oxidation since less fuel needed to heatoxygen reduces Cl₂ yield loss.

The process stream can be a recycle stream which is recycled back to oneor more steps of the process such as the step for reacting titaniumtetrachloride with oxygen or the step for contacting the gaseous productstream with silicon tetrachloride.

Applicants specifically incorporate the entire content of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

The description of illustrative and preferred embodiments of the presentinvention is not intended to limit the scope of the invention. Variousmodifications, alternative constructions and equivalents may be employedwithout departing from the true spirit and scope of the appended claims.

1. A process for making titanium dioxide, comprising: (a) reactingtitanium tetrachloride with oxygen by contacting the titaniumtetrachloride with the oxygen in an oxidation reactor under oxidationconditions to form a gaseous product stream containing titanium dioxide;(b) separating the titanium dioxide from the gaseous product stream toform a process stream; (c) analyzing the process stream to detect aconcentration of titanium tetrachloride in the process stream; (d)comparing the concentration of titanium tetrachloride detected in theprocess stream to a titanium tetrachloride aim point concentration; and(e) modifying the oxidation conditions to restore or maintain theconcentration of titanium tetrachloride in the process stream at the aimpoint.
 2. The process of claim 1 wherein step (a) further comprisescontacting the gaseous product stream with silicon tetrachloride underconditions effective for forming a treated product stream comprisingtitanium dioxide treated with a silicon-containing compound and whereinin step (b) the gaseous product stream is the treated product stream andthe titanium dioxide is separated from the treated product stream. 3.The process of claim 2 wherein step (c) further comprises analyzing theprocess stream to detect a concentration of silicon tetrachloride in theprocess stream and step (d) further comprises comparing theconcentration of silicon tetrachloride detected in the process stream toa silicon tetrachloride aim point concentration; and step (e) furthercomprises modifying the conditions for contacting the gaseous productstream with silicon tetrachloride to restore or maintain theconcentration of silicon tetrachloride in the process stream at thesilicon tetrachloride aim point.
 4. The process of claim 1 in which step(e) further comprises modifying the mixing conditions or the temperatureor both to reduce the concentration of titanium tetrachloride in theprocess stream to reach the titanium tetrachloride aim point.
 5. Theprocess of claim 3 in which modifying the conditions for contacting thegaseous product stream with silicon tetrachloride reduces theconcentration of silicon tetrachloride in the process stream to reachthe silicon tetrachloride aim point.
 6. The process of claim 1 in whichthe titanium tetrachloride aim point concentration is zero.
 7. Theprocess of claim 3 in which the silicon tetrachloride aim pointconcentration is zero.
 8. The process of claim 1 in which theconcentration of titanium tetrachloride is measured by an optical methodselected from the group consisting of transmission filter Infraredspectroscopy, transmission Fourier Transform Infrared spectroscopy,Raman spectroscopy, Near Infrared Spectroscopy and Ultravioletspectroscopy.
 9. The process of claim 1 in which the concentration oftitanium tetrachloride is measured in a frequency range of from 200 nmto 400 nm, 12,500 cm⁻¹ to 4000 cm⁻¹, and 4000 cm⁻¹ to 400 cm⁻¹.
 10. Theprocess of claim 3 in which the concentration of silicon tetrachlorideis measured by an optical method selected from the group consisting oftransmission filter Infrared spectroscopy, transmission FourierTransform Infrared spectroscopy, Raman spectroscopy, Near InfraredSpectroscopy and Ultraviolet spectroscopy.
 11. The process of claim 3 inwhich the concentration of silicon tetrachloride is measured in afrequency range of from 200 nm to 400 nm, 12,500 cm⁻¹ to 4000 cm⁻¹, and4000 cm⁻¹ to 400 cm⁻¹.
 12. The process of claim 1 wherein the oxidationconditions comprise a mixing step and an elevated temperature and theoxidation conditions are modified by adjusting the mixing or thetemperature or both.
 13. The process of claim 3 wherein the conditionsfor contacting the gaseous product stream with silicon tetrachloridecomprise a mixing step and an elevated temperature and the silicontetrachloride contacting conditions are modified by adjusting the mixingor the temperature or both.
 14. The process of claim 2 in which thetitanium dioxide treated with a silicon-containing compound is at leastpartially encapsulated with silica.
 15. A process for making titaniumdioxide, comprising: (a) reacting titanium tetrachloride with oxygen bycontacting the titanium tetrachloride with the oxygen in an oxidationreactor under oxidation conditions to form a gaseous product streamcontaining titanium dioxide; (b) contacting the gaseous product streamwith silicon tetrachloride under conditions effective for treating thetitanium dioxide with a silicon-containing compound to form a treatedproduct stream; (c) separating the treated titanium dioxide from thetreated product stream to form a process stream; (d) analyzing theprocess stream to detect a concentration of silicon tetrachloride; (e)comparing the concentration of silicon tetrachloride detected in theprocess stream to a silicon tetrachloride aim point concentration; and(g) modifying the conditions for contacting the gaseous product streamwith silicon tetrachloride to restore or maintain the concentration ofsilicon tetrachloride in the process stream at the silicon tetrachlorideaim point.
 16. The process of claim 15 further comprising analyzing theprocess stream to detect a concentration of titanium tetrachloride;comparing the concentration of titanium tetrachloride in the processstream to a titanium tetrachloride aim point concentration; andmodifying the oxidation conditions to restore or maintain theconcentration of titanium tetrachloride in the gaseous product stream orthe process stream at the titanium tetrachloride aim point.
 17. Theprocess of claim 15 in which the silicon tetrachloride aim pointconcentration is zero.
 18. The process of claim 16 in which the titaniumtetrachloride aim point concentration is zero.
 19. The process of claim15 in which the concentration of silicon tetrachloride is measured by anoptical method selected from the group consisting of transmission filterInfrared spectroscopy, transmission Fourier Transform Infraredspectroscopy, Raman spectroscopy, Near Infrared Spectroscopy andUltraviolet spectroscopy.
 20. The process of claim 15 in which theconcentration of silicon tetrachloride is measured in a frequency rangeof from 200 nm to 400 nm, 12,500 cm⁻¹ to 4000 cm⁻¹, and 4000 cm⁻¹ to 400cm⁻¹.
 21. The process of claim 16 in which the concentration of titaniumtetrachloride is measured by an optical method selected from the groupconsisting of transmission filter Infrared spectroscopy, transmissionFourier Transform Infrared spectroscopy, Raman spectroscopy, NearInfrared Spectroscopy and Ultraviolet spectroscopy.
 22. The process ofclaim 16 in which the concentration of titanium tetrachloride ismeasured in a frequency range of from 200 nm to 400 nm, 12,500 cm⁻¹ to4000 cm⁻¹, and 4000 cm⁻¹ to 400 cm⁻¹.
 23. The process of claim 15wherein the conditions for contacting the gaseous product stream withsilicon tetrachloride comprise a mixing step and an elevated temperatureand the silicon tetrachloride contacting conditions are modified byadjusting the mixing or the temperature or both.
 24. The process ofclaim 16 wherein the oxidation conditions comprise a mixing step and anelevated temperature and the oxidation conditions are modified byadjusting the mixing or the temperature or both.